Recently, Homogeneous-Charge Compression Ignition (HCCI) combustion in which gasoline fuel mixed with air is combusted by self-ignition inside a sufficiently compressed combustion chamber has attracted attention. HCCI combustion is a mode in which the mixture gas combusts at a plurality of positions simultaneously without flame propagation and, thus, has a higher combustion speed of the mixture gas than in SI combustion (spark-ignition combustion) which is adopted for general gasoline engines. Therefore, HCCI combustion is said to be significantly advantageous in terms of thermal efficiency. However, in a case of implementing HCCI combustion in an engine of an automobile for which improved thermal efficiency is desired, there are various issues to be solved and an engine which suitably performs the HCCI combustion has not been put into practical use. That is, while the engine mounted on the automobile greatly changes in operating state and its environmental condition, HCCI combustion has issues such as a combustion start timing of the mixture gas (a timing at which the mixture gas self-ignites) greatly varies due to external factors (e.g., atmospheric temperature) and control during a transient operation in which an engine load sharply changes being difficult.
Therefore, instead of combusting all of the mixture gas by self-ignition, it is proposed to combust a portion of the mixture gas by spark-ignition using a spark plug. That is, after forcibly combusting a portion of the mixture gas through flame propagation caused by spark-ignition (SI combustion), the remaining mixture gas is combusted by self-ignition (CI combustion). Hereinafter, such combustion mode is referred to as “SPCCI (SPark Controlled Compression Ignition) combustion.”
For example, JP2009-108778A discloses an engine adopting a similar concept to the SPCCI combustion. This engine causes flame propagation combustion by spark-igniting a stratified mixture gas which is formed around a spark plug by a supplementary fuel injection, and then performs a main fuel injection inside a combustion chamber warmed up by an effect of the flame propagation combustion, so as to combust the fuel injected in the main fuel injection through self-ignition.
The CI combustion of the SPCCI combustion occurs when a temperature inside a cylinder (in-cylinder temperature) reaches an ignition temperature of the mixture gas determined by a composition of the mixture gas. Fuel efficiency is maximized by causing the CI combustion when the in-cylinder temperature reaches the ignition temperature near a TDC of compression stroke (CTDC). The in-cylinder temperature increases as pressure inside the cylinder (in-cylinder pressure) increases. An increase in the in-cylinder pressure on the compression stroke when the SPCCI combustion is carried out is caused by two factors: compression work of a piston and the combustion energy of the SI combustion. If flame propagation of the SI combustion is not stable, increasing amounts of the in-cylinder pressure and the in-cylinder temperature resulting from the SI combustion decrease, and thereby it becomes difficult to increase the in-cylinder temperature to the ignition temperature. If the in-cylinder temperature does not fully increase to the ignition temperature, an amount of the mixture gas for the CI combustion decreases, which results in a large amount of the mixture gas combusts by the flame propagation of a long combustion period, or the CI combustion takes place when the piston descends considerably, and as a result, the fuel efficiency decreases. Thus, in order to stabilize the CI combustion to maximize fuel efficiency, it is important to stabilize the flame propagation of the SI combustion.
It may be possible to stabilize the flame propagation of the SI combustion by leaving hot burnt gas in the cylinder. However, if the hot burnt gas remains in the cylinder, the in-cylinder temperature excessively increases when the compression starts, which results in an excessive in-cylinder pressure rise when the CI combustion takes place near the compression top dead center, thereby inducing excessive combustion noise. In order to reduce combustion noise, the ignition timing may be, for example, retarded to delay the start timing of the CI combustion. However, if the ignition timing is retarded, since the CI combustion takes place when the piston descends considerably during the expansion stroke, fuel efficiency decreases.
Thus, since the SPCCI combustion is a novel combustion type, a configuration which can realize a suitable SPCCI combustion has not yet been found so far, i.e., a configuration to increase the ratio of the CI combustion while realizing the stable SPCCI combustion and reducing combustion noise to a tolerable level.